1. Field of the Invention
This invention relates to novel catalysts comprising phosphorus-vanadium mixed oxides or phosphorus-vanadium-co-metal mixed oxides and to processes for the manufacture and use of such catalysts. More particularly, this invention relates to such catalysts which are suitable for the oxidation of hydrocarbon feedstocks comprising benzene, butane, butenes, butadiene, or mixtures thereof to maleic anhydride and to their method of manufacture.
2. Prior Art
Maleic anhydride is of significant commercial interest throughout the world and is extensively used in the manufacture of alkyd resins. It is also versatile intermediate for chemical synthesis. Consequently, large quantities of maleic anhydride are produced each year to satisfy these needs.
In general, catalysts proposed for the oxidation of butane to maleic anhydride have been based upon vanadium and phosphorus. In U.S. Pat. No. 3,293,268, it is disclosed that the oxidation of butane to maleic anhydride can be performed in the presence of a phosphorus-vanadium-oxygen-containing complex catalyst. Though this catalyst is capable of oxidizing butane, it does not give sufficiently high yields. Yields of maleic anhydride of only 30 to 50 weight percent are reported. Various activators, stabilizers, and promoters have been disclosed in the prior art to improve the yields of maleic anhydride. References include U.S. Pat. Nos. 3,867,411; 3,832,359; 3,888,886; 4,002,650; 4,147,661; 4,149,992; 4,151,116; 4,152,338; 4,152,339; 4,403,943; 4,154,703; and British Application No. 2,019,839A. While the aforementioned prior art tends to bring about some improvement in the performance of the phosphorus-vanadium catalyst, there remains much room for improvement, particularly from the standpoint of high conversion, yield, and catalyst life. Other references of interest include U.S. Pat. Nos. 4,020,174; 4,094,816; 4,089,807; 3,296,282; 3,474,041, and British Patent No. 1,464,198. All of these references relate to catalyst regeneration and not to catalyst stability.
Also, U.S. Pat. Nos. 3,915,892 and 3,985,775 teach a process for preparing catalysts suitable for preparing maleic anhydride from n-butane comprising a mixed vanadium-phosphorus oxide wherein one of the process steps consists of heating the components to between 350.degree. C. (662.degree. F.) and 410.degree. C. (770.degree. F.) in an oxygen-containing gas. The function of this step is to remove water of hydration from the dihydrate of the mixed oxide of the vanadium and pentavalent phosphorus complex.